This is quite a large question to ask, 'How can my site scale to handle traffic?' and you won't get a comprehesive response on SO alone. There are books dedicated to this as well as careers. But if you've got some time before the rush and are a fast learner, here are areas to investigate few ways to help save your bacon:
- Use a CDN to serve your images--this will take a huge load off your webserver. Your situation is exactly what CDN technology was invented to address (by Akamai in case you asked in pub trivia, although they're a bit overpriced for normal people use)
- Utilize server caching the best as you can. I'm fairly sure Wordpress has some good caching plugins to memcache and such. I even think they've got file caching if you lack a better caching option. years ago I used a plugin called 'WP Super Cache'
- Move from one server to a load balanced pair
- Move you Mysql server off your webserver machine so your webserver and db aren't competing for resources. You can have many webserver hitting a single db server.
- Stick a reverse proxy cache like varnish in front of your site.
- Make sure your MySQL configuration is tweaked optimally for your needs and data size: Have the right size query cache, a good setting for innodb buffer pool size (if using innodb), etc...
- Consider moving to a nginx & php-fpm instead of apache and mod_php
Good luck, just from your question I'd really recommend getting some help if you've not got a lot of time before this event. Especially if this could be a large income boon for you or your site--ask anyone about how much adword revenue they could have made if they didn't crash that one day they were slashdotted...